Mortality and foraging rates of Argentine ant (Hymenoptera : Formicidae) colonies exposed to potted plants treated with fipronil

Individual colonies of Argentine ants, Linepithema humile (Mayr), were coll ected and maintained in plastic containers and randomly selected to receive a potted oleander (Nerium oleander L.) plant with one of eight different s oil treatments of fipronil, diazinon, or an untreated control. Foraging and mortality rates were observed in each colony. Reductions in the rates of a nts foraging on plants were similar with all treatments; however, the morta lity rates of the colonies varied among treatments. For example, 1 wk after exposure to treatments, the foraging rates on plants in all treatments exc ept the control had dropped to zero. Soil-mix treatments of fipronil had ki lled >90% of worker ants at 1 wk, whereas broadcast treatments of fipronil and diazinon killed <50% of workers. The soil-mix treatments of fipronil ki lled all queens in 4 wk, whereas broadcast treatments of fipronil took 8 wk to kill all queens. The diazinon broadcast treatment did not kill any quee ns. Only fipronil soil-mix treatments prevented ants from establishing colo nies in the pot. These results suggest that when studying pesticide effecti veness against Agrentine ant in the field, equivalent short-term reductions in foraging rates should not be assumed to indicate equivalent mortality o f ant colonies.